For many military electronic systems that receive and/or transmit electromagnetic radiation, broadband (frequency independent) performance is advantageous or even required. One common class of broadband antennas uses a planar spiral configuration--either archimedean or logarithmic.
For many of these applications, the operating environment requires the antenna structure to be protectively enclosed within a radome made of a dielectric material. Thus, the antenna is required to "look through" the radome.
Typically, although these antennas are bi-directional (radiating forward and backward), forward-looking unidirectional performance is required to avoid undesirable, reflections from the cavity backing. Thus, these antennas are backed by an absorber filled cavity that absorbs radiation on the backside of the antenna plane.
Broadband absorber-cavity-backed planar spiral antennas are capable of providing symmetrical fixed beam radiation patterns over multi-octave bandwidths when operating in a free space environment, i.e., without the radome structure. When placed within the radome, performance is usually degraded due to distortions introduced by the radome, with the degree of degradation in symmetrical broadband performance being dependent upon radome configuration.
If a broadband planar spiral antenna is placed within a hemispherical-shaped radome, the broadband symmetrical performance, although somewhat degraded, is still acceptable for many applications. However, many applications, including most of those for aircraft or missile radar systems, require that radome shape be dictated by aerodynamic considerations (both in terms of streamlining and structural integrity) rather than to accommodate antenna structures. For many of these applications, a hemispherical radome is impractical.
An example of an aerodynamically-shaped radome is the sharply-tapered ogive geometry, commonly used for aircraft/missile applications. Locating a broadband planar spiral antenna within such a streamlined radome increases radome-induced distortions, and therefore, further degrades symmetrical broadband performance over that for hemispherically-shaped radomes. Nevertheless, for some applications, even this increased level of broadband symmetrical performance degradation is acceptable.
Unfortunately, the nose of an aircraft or missile is a prime antenna location that cannot accommodate all of the desired antenna structures, even where collocation techniques are used to permit the sharing of a common antenna aperture. One alternative location for an antenna structure that still allows forward looking operation is the leading edge of a wing.
The leading edge geometry of a wing is such that the aerodynamic streamlining requirements for a radome with a leading edge profile are even more severe than for nosecone radomes. Specifically, a radome with a corresponding leading edge profile causes even more distortion than the symmetrical nosecone radome because its folded, wedge-shaped geometry is not only severely streamlined, but is also highly asymmetrical.
Because of the severely tapered geometry of a leading edge radome, a planar antenna must of necessity be located back from the radome shell apex, leaving a forward cavity between the antenna structure and the radome. As a result, the antenna unavoidably receives not only the direct-path signal from the source, but also a multi-path signal reflected from the internal wall of the radome. These internal wall reflections are more severe in the case of the asymmetrical leading edge geometry than for a symmetrical geometry (such as an ogive) because they are concentrated in the elevation dimension. As a result, the elevation radiation pattern is severely distorted, and for most applications is essentially useless.
The degradation in pattern performance for an antenna structure located in a leading edge radome severely restricts the utility of a broadband symmetrical antenna structure for Direction Finding and most other applications. Thus, heretofore, broadband planar spiral (or other symmetrical) antenna structures have not been located in the leading edge of an aircraft wing.
One approach to solving the problem of locating a broadband antenna in the leading edge of an aircraft wing is disclosed in U.S. Pat. 4,697,192 "Two Arm Planar/Conical/Helix Antenna", issued Sep. 29, 1987 to the same inventors and assignee as the invention disclosed and claimed herein. This patent addressed the leading-edge radome problem by developing a planar/conical/helix antenna structure to fit farther forward within the leading edge profile than would be possible with a planar antenna with comparable performance. However, this antenna is still required to "look through" the radome; in particular, the planar/conical/helix antenna cannot be made integral with the leading edge radome shell. Thus, while its broadband radiation pattern is significantly improved over that which would be obtained from a conventional planar spiral antenna, it still is subject to substantial pattern degradation due to radome induced problems.
Accordingly, a need exists for a broadband antenna structure whose profile conforms to the asymmetrical folded radome shell, such as for incorporation into the leading edge of an aircraft wing. Preferably, the antenna would provide substantially symmetrical performance for both azimuth and elevation, with pattern and gain performance similar to that available from conventional planar broadband antennas.